1. Field of the Invention
This invention relates to an infrared detector, particularly to an infrared detector having high security to overcome disadvantages of a conventional infrared detector which is liable to be disabled and lose its function.
2. Description of the Prior Art
A conventional infrared detector generally includes a light emitter, and a light receiver combined together. In operation, the light emitter and the light receiver are positioned to face each other and are spaced apart with a preset distance within an effective scope. The light emitter emits an infrared light beam with a code, such as a lane-style protective light beam, to the light receiver. When, anyone such as a thief should walk through the gap between the light emitter and the light receiver to interrupt the infrared light beam from being received by the light receiver, a protective appliance such as a threatening lamp or an alarm may be automatically triggered to call attention to the interruption detected by the infrared detector. Many buildings use this kind of infrared detector for security. Further, conventional infrared detectors have a single-lane, a double-lane or a multi-lane light beam. The double-lane light beam infrared detectors are popularly used, as they can prevent false alarms caused by a falling leaf or the like The operation principle of double-lane light beam infrared detectors includes using two infrared light emitting units for the light emitter and two light receiving units for the light receiver, with the units positioned spaced apart with a proper distance. If only one of the two units should be interrupted by something, the light receiver does not function to turn on a lamp or an alarm so as to prevent the light detector from functioning as a result of interruption caused by small things such as falling leaves, insects, birds, etc. Only when the two lanes of the infrared light beam are at the same time interrupted, the light detector will operate to turn on a lamp or an alarm.
Though the conventional infrared detectors can attain the purpose of using a double-lane infrared light beam for detecting intrusion, they are quite unsafe in practical use. FIG. 1 shows how the conventional infrared detector functions, with the conventional infrared detector including double-lane infrared light emitting units 1A and 1B of a light emitter 1 giving out two lanes of an infrared light beam, and including two light receiving units 2A and 2B receiving the infrared light beam coming from the light emitting units 1A and 1B. However, the conventional infrared detector of FIGS. 1 and 2 has the following disadvantages.
1. As to its effect: If a person wants to disable the conventional infrared detector, he can easily do it by using another light emitter 3 of the same source or a different source. The other light emitter 3 can be utilized to give out a double-lane infrared light beam from its two light emitting units 3A and 3B at a shorter distance than that between the original light emitter 1 and the original light receiver 2. As shown in FIG. 1, the other light emitter 3 can be oriented with only one of the two lanes of the light beam of the two emitting units 3A or 3B received by one of the light receiving units 2A or 2B, or as shown in FIG. 2, can be oriented with the two lanes of the light beam given out by the two emitting units 3A and 3B both received by the two light receiving units 2A and 2B. In either case, the effective distance of the infrared detector may become only the distance represented by L in FIGS. 1 and 2. The distance represented by L1 in FIGS. 1 and 2 between the light emitting units 1A and 1B and 3A and 3B loses its protective effect to permit a thief to pass across the light beam between the light emitter 1 and the light receiver 2 without triggering the light receiver 2 to function to turn on a lamp or an alarm. In other words, the conventional infrared detector is nominal and unsubstantial, being prone to be disabled by another infrared light emitter 3.
2. As to difficulty: Even if a would-be thief can not make out which is the light receiver 2, the would-be thief can use two sets of the other light emitters 3 to emit infrared light beams to both the light emitter 1 and the light receiver 2 thereby making the light receiver 2 to lose function so as to allow unauthorized access.
3. For keeping secret: The information used between the light emitter 1 and the light receiver 2 of the conventional infrared detector is a constant single code, nearly impossible to keep secret.
4. As to security: The conventional infrared detector cannot maintain protection against illegal use of other infrared emitters, and thus is not safe to use.